Atrial Fibrillation (AF) is a common disorder related to abnormal beating of the heart. Specifically, instead of beating in a normal rhythm, the heart's two small upper chambers (the atria) quiver such that blood is not completely pumped from the atria. The residual blood may form a clot with devastating consequences. For example, if a piece of a blood clot formed in the atria leaves the heart and becomes lodged in an artery in the brain, a stroke may result. In fact, about 15 percent of strokes are related to atrial fibrillation.
It is well documented that atrial fibrillation (AF), either alone or as a consequence of other cardiac disease, continues to persist as the most common type of cardiac arrhythmia. In the United States alone, AF currently affects an estimated two million people, with approximately 160,000 new cases being diagnosed each year. The cost of treatment for AF is estimated to be in excess of $400 million worldwide each year.
Post operative AF is a significant problem for hospitals worldwide with no effective solution. AF is the most common morbidity event after coronary bypass grafting. It has been estimated that the incidence of AF following coronary artery bypass graft (CABG) surgery is between 25% and 40%. The rate is even higher for patients undergoing valve surgery either alone or in combination with CABG surgery. Although the AF may resolve itself within the first ten days following surgery, the problem is associated with high levels of morbidity during the post operative phase and can increase the cost of hospital stays by $20,000 or more.
Although pharmacological treatment is available for AF, the treatment is far from perfect. For example, certain antiarrhythmic drugs, like quinidine and procainamide, can reduce both the incidence and the duration of AF episodes. Yet, these drugs often fail to maintain sinus rhythm in the patient. Cardioactive drugs, like digitalis, Beta blockers, and calcium channel blockers, can also be given to control AF by restoring the heart's natural rhythm and limiting the natural clotting mechanism of the blood. However, antiarrhythmic drug therapy often becomes less effective over time. In addition, while drug therapy may mitigate the incidence of AF once it develops, it does nothing to prevent the problem from occurring.
Some investigators have attributed the cause of post-operative AF to an imbalance within the autonomic nervous system of the heart. The autonomic nervous system includes parasympathetic and sympathetic nerves. The parasympathetic pathway acts through a branch of the vagus nerve, which enters the heart near the ascending aorta. The sympathetic pathway acts through a nerve bundle called the cardiac plexus, which enters the heart within the Ligament of Marshall (LOM) as well as at other locations proximal to the cardiac plexus. The LOM is a fold of the pericardium from a developmental vestige of the left primitive veins, near the left atrial appendage. Under normal conditions, there is a balance between the sympathetic and parasympathetic stimulation of the heart. However, following surgery, there is an imbalance in the autonomic system resulting in an over-stimulation of the sympathetic pathway and/or an under-stimulation of the para-sympathetic pathway. This imbalance, due to the temporarily depressed parasympathetic activation following surgery, possibly due to surgical trauma, edema and general side effects of cardiac surgery, could likely result in post operative atrial fibrillation.
Attempts have been made during surgery to prevent post-operative AF. For example, surgical denervation methods remove the nerve supply to tissue by cutting or crushing the axons of nerve cells. On exemplary process, ventral cardiac denervation, entails the removal of nerves around large vessels at the base of the heart that run from the right side of the superior vena cava and end at the level of the midportion of the anterior pulmonary artery. The dissection starts at the right side of the pericardial cavity and the superior vena cava is completely dissected and separated from the right pulmonary artery. Then the fat pad around the aorta is dissected. Finally, all of the fatty tissues in the aorta-pulmonary groove and the inner half of the adventitia of the anterior pulmonary artery up to the left border are cut.
Another technique intended to prevent non post-operative AF is radiofrequency catheter ablation, wherein a catheter having an electrode at its tip is guided to an area of heart muscle having an accessory pathway. The catheter is placed at the exact site inside the heart where cells give off electrical signals that stimulate the abnormal heart rhythm. Radiofrequency energy transmitted through the pathway kills heart muscle cells in the specific area. Therefore, that area can no longer conduct the extra impulses that caused the rapid heartbeats.
Although radiofrequency ablation may be effective in some patients when medications don't work, the burst of radiofrequency energy destroys the tissue that triggers the abnormal electrical signals. Further, radiofrequency ablation techniques may lead to injury of contiguous structures, the development of thrombus and the risk of pulmonary vein stenosis. Finally, atrial pacemakers have been used as a treatment of AF. Pacemakers are implanted under the skin to regulate the heart rhythm of the patient. This, and the aforementioned procedures are invasive and non-reversible and place patients exhibiting symptoms of post operative atrial fibrillation at unnecessary risk.
In view of the above treatment limitations, it would be desirable to provide a reversible method and system of treating the nerves of the heart in order to reduce or eliminate sympathetic nerve stimulation to the heart in order to reduce the likelihood of postoperative atrial fibrillation.
Also in view of the above, it would also be desirable to provide a permanent method and system of treating the nerves of the heart without causing significant damage to adjacent structures thereby limiting the risk of pulmonary vein stenosis, thrombus formation or endocardial disruption.